“Conversely, systemic yohimbine (1 mg/kg) attenuated the effects of central etorphine , and diminished potentiation of etorphine by the alpha 2 agonists.”
“Prior microinjection of clonidine (5 micrograms) or guanfacine (5 micrograms) into the locus coeruleus (LC) reduced the intensity of central etorphine antinociception whereas central yohimbine (20 micrograms) pretreatment increased peak antinociceptive activity and prolonged the duration of etorphine.”